The growth of semiconductor III-V compounds by chemical vapor deposition (CVD) using organometallics and hydrides as elemental sources has recently developed into a viable process with many potential commercial applications. The metallo-organic chemical vapor deposition (MOCVD) process, based on the pyrolysis of alkyls of group-III elements in an atmosphere of the hydrides of group-V elements, is a common growth technique because it is well adapted to the growth of submicron layers and heterostructures.
Open-tube flow systems are used at atmospheric or reduced pressures in producing the III-V alloys. The process requires only one high-temperature zone for the in situ formation and growth of the semiconductor compound directly on a heated substrate.
FIG. 2 shows the x-y compositional plane for quaternary III-V alloys matched to an InP substrate at 300 K. The x-y coordinate of any point in the plane gives the compositions of the alloys. The curved contour lines correspond to curves of constant energy gap (E.sub.g). The straight black contour lines correspond to curves of constant lattice parameters. The black line labeled GaInP corresponds to alloys having a lattice constant matching that of GaAs 5.653 .ANG.. The cross-hatched region corresponds to alloys having indirect band gaps.
Low pressure (LP-) MOCVD growth method offers an improved thickness uniformity and compositional homogeneity, reduction of autodoping, reduction of parasitic decomposition in the gas phase, and allows the growth of high-quality material over a large surface area. The LP-MOCVD technique has been successfully used to grow Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y (0.ltoreq.x.ltoreq.0.47 and 0.ltoreq.y.ltoreq.1) lattice matched to InP for the complete compositional range between InP (.lambda.=0.91 .mu.m, E.sub.g =1.35 eV) and the ternary compound Ga.sub.0.47 In.sub.0.53 As (.lambda.=1.67 .mu.m, E.sub.g =0.75 eV). GaInAsP alloys, which are potentially useful materials both for heterojunction microwave and optoelectronic device applications can be grown by liquid-phase epitaxy (LPE), molecular-beam epitaxy (MBE), conventional vapor-phase epitaxy (VPE), as well as MOCVD.
The disadvantages of LPE include growth problems with GaInAsP alloys for .lambda.&gt;1.4 .mu.m and potential nonuniform growth as well as melt-back effect. Molecular-beam epitaxy is a very expensive and complex process, and difficulties have been reported with p-type doping and with the growth of phosphorus-bearing alloys. Vapor-phase epitaxy disadvantages include potential for hillock and haze formation and interfacial decomposition during the preheat stage.
The technique of LP-MOCVD is well adapted to the growth of the entire composition range of GaInAsP layers of uniform thickness and composition that is lattice matched to GaAs over areas of more than 10 cm.sup.2. This results first from the ability of the process to produce abrupt composition changes and second from the result that the composition and growth rate are generally temperature independent. It is a versatile technique, numerous starting compounds can be used, and growth is controlled by fully independent parameters.
Growth by MOCVD takes place far from a thermodynamic equilibrium, and growth rates are determined generally by the arrival rate of material at the growing surface rather than by temperature-dependent reactions between the gas and solid phases. In contract to LPE growth, it has been found that during MOCVD growth of a double heterostructure, GaAs can be grown directly on GaInAsP with no disturbance of the active layer, i.e., there is no effect equivalent to melt-back.
One of the key reasons for the usefulness of this method is the possibility of obtaining high-purity and therefore high-mobility Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y. As long-wavelength 1.0-1.65 .mu.m GaInAsP electro-optical devices become more widely used, motivated by low fiber absorption and dispersion, high transmission through water and smoke, and greatly enhanced eye safety at wavelengths greater than 1.4 .mu.m, LP-MOCVD offers the advantages of smooth uniform surfaces, sharp interfaces (lower than 5 .ANG. for GaInAsP/GaAs), uniformly lower background doping density, and economy of scale for large-area devices.
Recent studies have shown the feasibility of using InGaAsP/GaAs heterostructures as diode lasers. The diodes can be used successfully for solid state laser pumping and can be interchanged with lasers based on AlGaAs/GaAs heterostructures.
Laser diodes emitting at 808 nm wavelength are important sources for the pumping of YAG:Nd lasers. AlGaAs/GaAs lasers are commonly used for this purpose, but there are several problems with these structures: Oxidation of AlGaAs layers which makes further regrowth and device fabrication difficult; higher growth temperature which may not be compatible with monolithic integration; and the presence of dark line defects and dislocation migration which can cause degradation in performance. Most of these problems can be attributed to the presence of Aluminum. High-power quantum well lasers based on liquid phase epitaxy (LPE) grown GaInAsP/GaAs structures do demonstrate characteristics competitive to the best existing AlGaAs/GaAs separate confinement heterostructure-single quantum well (SCH-SQW) lasers but, as stated above, growth by LPE present several major disadvantages.